1. Field of the Invention
The invention relates to a multilayered substrate for a semiconductor device. More particularly, the invention relates to a multilayered substrate formed of a plurality of sets of a conductor layer and an insulation layer, i.e., a laminate of alternate conductor and insulation layers, and having a face for mounting semiconductor element thereon and another face for external connection terminals, the face for mounting semiconductor device being provided with pads through which the substrate is connected to a semiconductor element to be mounted thereon, and the face for external connection terminals being provided with pads through which the substrate is connected to an external electrical circuit. The invention also relates to a method of manufacturing such a multilayered substrate.
2. Description of the Related Art
A typical multilayered substrate for semiconductor device, which may be simply called a multilayered substrate herein, is illustrated in FIG. 19. This multilayered substrate 100 has a core substrate 102 made of resin, and an upper layered section 106a and a lower layered section 106b provided on the respective sides of the core substrate 102, each of the upper and lower layered sections 106a and 106b having layered wiring lines 104 of a conductor, and the wiring lines 104 of the upper and lower layered sections 106a and 106b being electrically connected through a through hole via 110 piercing through the core substrate 102. Also in each of the upper and lower layered sections 106a and 106b, wiring lines 104 on both sides of an insulation layer of resin are electrically connected through a via 118 piercing the insulation layer.
In the outermost layer of the upper layered section 106a, pads 124 are provided which are electrically connected with an electrode terminal 122 of a semiconductor element 120 to be mounted on the multilayered substrate 100. The outermost layer of the upper layered section 106a is covered by a solder resist 126 except for the pads 124.
In the outermost layer of the lower layered section 106b, pads 130 are formed to which a solder ball 128, as an external connection terminal, is to be bonded. The outermost layer of the lower layered section 106b is also covered by a solder resist 132 except for the pads 130.
The multilayered substrate for semiconductor element shown in FIG. 19 can be produced by a build-up process illustrated in FIGS. 20A to 20F. According to the build-up process, the upper and lower layered sections 106a and 106b shown in FIG. 19 are simultaneously formed. For this reason, FIGS. 20A to 20F show only the formation of the upper layered section 106a, omitting the formation of the lower layered section 106b. 
In the process shown in FIGS. 20A to 20F, a core substrate of resin 102 having a copper foil 100 on each side is first pierced by a means such as a drill to be provided with through holes. A copper film layer is then formed on the inside wall of the through hole by electroless plating and, as required, by subsequent electrolytic plating, to have a desired thickness and form a through hole via 110 (FIG. 20A).
The copper foil 100 of the core substrate 102 is then subjected to a subtractive process to form wiring lines 104 and pads 105, which are formed as part of the wiring line (FIG. 20B). A film 106 of polyimide, which is a thermosetting resin, having one face provided with a copper foil 108 thereon is adhered to the core substrate 102, with the face provided with the copper foil 108 being upwardly faced (FIG. 20C). In the step of adhesion of the polyimide film 106, a resin, such as a polyimide resin, is filled in the through hole vias 110. The film 106 with the copper foil 108 is then pierced by a laser beam to form holes 107 for the formation of vias, the hole extending to the underlying wiring line 104 to expose the pad 105 (FIG. 20D).
A copper layer 112 is then formed on the inside wall of each of the holes 107 to electrically connect the pad 105 of the wiring line 104 and the copper foil 108 (FIG. 20E). The copper layer 112 is formed by covering the copper foil 108 by a resist film 114 to leave the holes 107 uncovered, as shown in FIG. 20E, and forming a copper film layer, having a certain thickness, only on the inside wall of each hole 107 by electroless plating, sputtering or the like and, as required, by subsequent electrolytic plating.
The resist film 114 is then removed, and the copper foil 108 is subjected to a subtractive process to form wiring lines 116 (FIG. 20F). The wiring line 116 thus formed is electrically connected with the underlying wiring line 114 through a via 118 penetrating the resin layer 106.
By repeating the steps illustrated in FIGS. 20C to 20F, the multilayered substrate for a semiconductor, shown in FIG. 19, can be obtained.
Using the multilayered substrate 100 as shown in FIG. 19, a semiconductor element 120 having a high density can be mounted thereon. In the upper layered section 106a of the multilayered substrate 110 of FIG. 19, however, since the respective layers are successively formed upwardly from the surface of the core substrate 102, the outermost layer, on which a semiconductor element 120 is to be mounted, is prone to have a less flat surface (i.e., a less even surface) due to accumulation of unevenness of the underlying layers. Consequently, when a semiconductor element 120 is mounted on the mounting face of the multilayered substrate of FIG. 19 in a flip chip fashion, for example, some of electrode terminals 122 of the semiconductor element 120 may be left unconnected to the pads 124 of the outermost layer of the substrate 100.
In a build-up process illustrated in FIGS. 20A to 20F, the respective layers of the upper and lower layered sections 106a and 106b are simultaneously built up on the respective sides of the substrate, as described above, because if layers are formed only on one side of the substrate, the resultant substrate may be warped. Consequently, even if it is sufficient for a substrate to have only upper layered section, it is required to form the lower layered section to prevent the substrate from being warped, which makes the resultant substrate thicker.
An object of the invention is to provide a multilayered substrate for semiconductor element having a face for mounting semiconductor element thereon which is as even as possible, and having a thickness as small as possible.
Another object of the invention is to provide a method of manufacturing such a multilayered substrate.
The inventors have found that alternately forming wiring line layers and insulation layers successively from the side for mounting a semiconductor element to the side for external connection terminals makes it possible to produce a multilayered substrate without using a core substrate, the produced multilayered substrate having a face for mounting semiconductor element thereon which is substantially even or is as even as possible.
Thus, in one aspect, the invention provides a substrate of multilayered structure having a plurality of sets of an insulation layer and a wiring line layer, and having one face for mounting a semiconductor element thereon and the other face on which external connection terminals are to be provided, the face for mounting a semiconductor element being provided with pads to be bonded to an electrode terminal of the semiconductor element, the other face being provided with pads to be bonded to an external connection terminal, such as a terminal formed of a solder ball, and the wiring line layers on both sides of an insulation layer being connected with each other by vias piercing the insulation layer, wherein the surfaces of the pads to be bonded to an electrode terminal of a semiconductor element are flat and are in the same plane. In this multilayered substrate of the invention, the surfaces of the pads to be bonded to an external connection terminal may be or may not be flat, and may be or may not be in the same plane.
Preferably, the via is made up of a metallic layer formed along the inside wall and bottom of a hole piercing through the insulation layer.
Preferably, the via is made up of a column of metal filled in a hole piercing through the insulation layer.
More preferably, the column of metal has end faces which are flat and are respectively connected with the wiring lines on both sides of the insulation layer.
Preferably, the pad to be bonded to an electrode terminal of the semiconductor element has a surface layer of metallic material which is melted at a temperature used to bond the pad to the electrode terminal of the semiconductor element.
More preferably, the surface layer is formed of a solder material.
Preferably, the pad to be bonded to an electrode terminal of the semiconductor element is formed of two or more layers of metals, and the layer directly in contact with the electrode terminal of the semiconductor element is based on a noble metal.
More preferably, the noble metal is gold.
The substrate of multilayered structure of the invention may comprise a frame member in a peripheral region of the face for mounting a semiconductor element thereon.
Preferably, the frame member is made of copper, aluminum, or stainless steel.
The substrate of multilayered structure of the invention may also comprise a thin film capacitor incorporated therein.
In another aspect of the invention, the substrate of a multilayered structure of the invention can be manufactured by a method in which a plurality of sets of an insulation layer and a wiring line layer are successively formed from the side of the multilayered substrate at which the face for mounting a semiconductor element is located to the other side of the multilayered substrate at which another face of the multilayered substrate is located, while wiring line layers on both sides of each insulation layer are connected with each other by vias piercing the insulation layer.
Thus, the method of the invention can comprise the following steps:
(a) providing a metal sheet,
(b) forming, on the metal sheet, the pads to be bonded to an electrode terminal of the semiconductor element,
(c) forming a layer of insulating material on the metal sheet so as to cover the pads formed thereon,
(d) piercing the layer of insulating material to form holes which expose the end face of the pad,
(e) forming a patterned metal layer to provide a layer of wiring lines which includes wiring lines formed on the layer of insulating material and vias, the via connecting the pad in the hole with the wiring line on the layer of insulation material,
(f) forming a layer of insulation material to cover the layer of wiring line,
(g) piercing the layer of insulating material formed in step (f) to form holes which expose part of the underlying wiring line,
(h) forming a patterned metal layer to provide a layer of wiring lines which includes wiring lines formed on the layer of insulating material and vias, the via connecting the part of the wiring line exposed in step
(g) with the wiring line on the layer of insulation material,
(i) repeating steps (f) to (h) until the predetermined number of sets of an insulation layer and a wiring line layer is obtained, and
(j) removing the metal sheet from the sets of an insulation layer and a wiring line layer formed thereon.
Preferably, the metal sheet is removed by etching.
Preferably, the metal sheet is made of cooper, aluminum, or stainless steel.
Preferably, the metal sheet has a seed layer on a surface on which the sets of an insulation layer and a wiring line layer are to be formed, the seed layer being made up of two or more sublayers of different metal materials, and the sublayer in contact with the metal sheet being formed of a metal material which is not etched or is substantially not etched by an etchant for the metal sheet.
More preferably, the metal sheet is formed of copper, and the seed layer is made up of a sublayer, in contact with the metal sheet, of chromium or titanium, and another sublayer of copper.
A metal sheet provided with a layer of insulating material on a surface may be used, and the pads to be bonded to an electrode terminal of the semiconductor element may be formed by piercing through the layer of insulating material to form holes exposing the surface of the metal sheet, and then electrolytically plating the exposed surface of the metal sheet to form a plated layer in the hole.
Preferably, the metal sheet is partially removed so as to leave a frame member on a peripheral region of the multilayered substrate.
Preferably, the formation of vias in steps (e) and/or (h) is carried out by forming a metal layer along the inside wall and bottom of the hole piercing through the insulation layer.
Preferably, the formation of vias in steps (e) and/or (h) is carried out by making a column of metal material filling the hole piercing through the insulation layer.
Preferably, the column is made by forming a layer of metal to cover the insulation film and fill the holes piercing through the insulation film, and then abrading the formed layer of metal to expose the insulation film and leave the metal filling the hole.
Preferably, the pad to be bonded to an electrode terminal of the semiconductor element is formed so as to have a surface layer of metallic material which is melted at a temperature used to bond the pad to the electrode terminal of the semiconductor element.
More preferably, the surface layer is formed of a solder material.
Preferably, the pad to be bonded to an electrode terminal of the semiconductor element is formed so as to have two or more layers of metals, the layer to be directly in contact with the electrode terminal of the semiconductor element being based on a noble metal.
More preferably, the noble metal is gold.